FR 2639561 discloses a metal can and a method of manufacturing the same, the metal can comprising an internal annular flange to which a peelable lid can be heat-sealed in order to hermetically close the can.
In that application a peelable lid has a heat sealable layer which is used to hermetically seal the can. An alternative heat sealing could involve providing adhesive around the upper surface of the flange and/or around the under surface of the peelable lid, heating the flange and applying downward pressure.
For some markets, the type of metal can described in FR 2639561 may be perfectly adequate. However, for more specialised markets, for example the infant formula market (i.e. metal cans that are used to store baby milk powder), the safety of babies and young children can be at risk, and so the cans, including the seals, are required to have very high performances even in the most hostile of environments. Therefore these metal cans are required to undergo a series of stringent tests before they are deemed as safe to use in the marketplace. For example, it is desirable that a metal can that is to be used to store infant formula powder does not lose its hermetic seal with the peelable lid, even when stored at high ambient temperatures such as 45° C. for periods in excess of 3 months with a pressure difference from inside to outside of the can, for example of 700 mbar (70 kPa). Typically the external pressure is standard (ambient) air pressure and the internal pressure is negative, often referred to as a “vacuum”.
Under such conditions, it has been found that metal cans made in accordance with those described in the prior art are prone to suffering from “creep” in the seal. Creep is the tendency for the peelable lid to slowly move from its position on the flange, due for example to pressure differences, particularly at high temperatures. This can reduce the effectiveness of the seal between the peelable lid and the flange, and in some cases may cause the seal to fail completely. Metal cans that suffer from creep in this way cannot be used in the infant formula market as the seal is not deemed to seal the product to a high enough standard.